Thermoplastic thermoformable composite material and method of forming such material

ABSTRACT

A thermoplastic thermoformable composite material for shaping and stretching into a desired form without voids and holes, including a core formed by at least one layer of chopped fibers enveloped and impregnated by thermoplastic material to form a fabric layer. Layers of thermoplastic material respectively positioned on opposite sides of the fabric layer core, and the layers of thermoplastic material provided with a sufficient thickness to flow into and heal any voids or holes formed in the core as the composite material is shaped and stretched into a desired form.

BACKGROUND OF INVENTION

1. Field of the Invention

This invention relates to a thermoformable thermoplastic composite sheetmaterial which is strong and light and which is capable of beingthermally deformed and stretched to any desired shape without buckling,rippling or tearing of the composite material.

2. Description of Prior Art

It is often desirable to form a sheet of plastic material into a complexshape without any rippling, buckling, or tearing of the material. It isalso desirable to make such sheet as light (from a weight standpoint),thin and strong as possible. It is also desirable to form a sheet withthe above characteristics into the complex shape in a relatively simpleprocess so that the costs of providing the complex shape in the sheetare low and so that the yield of the final product is high.

A sheet with the properties discussed may have a wide variety ofdifferent uses. For example, it may be shaped to form a container, orthe sides of a container such as used in air cargo. Other uses may be toform components or portions of a boat or vehicle. Although the exampleshave been limited to uses in transportation, this is only one of anumber of different fields or areas in which such material can be used.

Thermoplastic materials have been used to provide sheets with theproperties discussed above. To obtain a desired shape, a mold isprovided with the desired shape. The thermoplastic material is thenformed into the desired shape by applying heat and pressure to thethermoplastic material to move the material into the mold and to havethe material adopt the configuration of the mold. Suitable materials forforming into complex shapes may be any thermoplastic polymer includingacrylics, ABS, polyolefins, nylon and polycarbonates and the engineeringthermoplastic such as polysulfones and PBT (polybutyleneteraphthalate,and the new thermoplastic resins such as PEEK (polyetheretherketone),PEKK (polyetherketoneketone) etc.

Thermoplastic materials are advantageous because they can be reshaped ifit is desired to change the configuration somewhat after the materialhas been initially shaped through the application of heat and pressure.However, thermoplastic materials are distinctly disadvantageous in thatthey have to be provided with a considerable thickness in order toprovide the necessary rigidity. For example, thicknesses of 0.120" to0.200" may be required. Unfortunately, such thick materials are heavyand weight is a disadvantage in transportation. Furthermore, thethermoplastic materials are not as durable and resistant to breaking aswould otherwise be required.

Thermosetting materials that contain reinforcing fibers have been usedto provide complex shapes. An advantage of a thermosetting material isthat they can be made quite thin to obtain the desired shape. Forexample, the thickness of the thermosetting material may be in the order0.125" and be as stiff as material 0.200". One problem with shapingthermosetting materials into complex shapes is that the thermosettingmaterials have to be shaped properly the first time. The reason is thatthe thermosetting materials cannot be reshaped after they have beenheated to a temperature for initially shaping the material. This problemhas limited the use of thermosetting material to provide large complexshapes. "Another problem often is that the weight of the thermosettingmaterial tends to be heavy and irregular even though the part is thin."

Fiberglass is a thermosetting material that uses styrene basedpolybutylene resin glass as a reinforcement. "Open Molding" is used tofabricate large "fiberglass" parts for businesses traditionally needingmodest production volumes. Part costs for such open molding are growingrapidly because throughput is slow, labor content is high and openmolding is unfriendly to man and his environment. One reason fabricatorskeep using "fiberglass" is familiarity. It was developed in the late40's. Raw materials are inexpensive and make good structural parts.Tooling is cheap and is made rapidly. Another reason for the use of"fiberglass" is the lack of viable alternatives. Unfortunately, risingpart costs, worker's health and environmental issues are fundamentalproblems with "fiberglass". These problems can not easily be resolved.

Matched metal mold technology which uses similar styrene containingmaterials is available. It uses heated molds to rapidly cure material.This reduces health and environmental issues and eliminates hazardouswaste. However, it is too expensive to be competitive at productionvolumes needed for open molded parts.

The fiberglass technology is based on open air curing (polymerizing) ofstyrene. Parts are made by painting a styrene containing polyester resinonto glass fabric. This fabric/resin layer is built up on a mold surfaceto the needed thickness. The parts harden within 4 hours.

Part fabrication using fiberglass technology is labor intensive andcosts are rising. The labor cost can be and greater of the cost ofgoods. The fiberglass process is unfriendly to workers and theirenvironment. Styrene is volatile and the chemically reactive materialmix effects workers as follows:

1. Workers Health

Worker's Compensation Insurance rates are higher for workers, who inhalestyrene daily. It is known that aromatic chemicals like styrene damageliver and kidney. Styrene is a possible carcinogen and is on the IRACwatch list.

2. Air Pollution

Styrene pollutes the air. Emissions are regulated by permit. Companygrowth can be stopped if all permit capacity for a given plant is inuse.

3. Hazardous Waste Disposal

The chemically reactive mix forms a hazardous waste. Disposal fees arerapidly increasing as dump sites get scarce.

Thermoforming parts from glass or other fiber reinforced thermoplasticsheet would be an alternative to "fiberglass technology", but this newconcept has unresolved technical problems. The thermoforming aspect isan old fabrication technology which started in the 1950's. Thistechnology is used to form parts in modest production volumes. It offerslow labor costs, fast forming cycles and low cost tooling. It usesvacuum to form parts from heat softened thermoplastic sheet that isclamped in a frame on top of the mold. Plastics include polyolefines,acrylics, ABS, polycarbonate nylons, etc. The problem is with materials.The fiber reinforced thermoplastic sheets available today, are notadequate for the job of thermoforming acceptable parts.

Todays Sheet Products

At first glance, the extrusion process, for making general purposethermoplastic into sheet, would appear to be a feasible way of makingfiberglass filled sheet. This is not often done because glass fiber isabrasive and causes excessive wear on equipment. Also this processsignificantly reduces fiber length in the extruded sheet and thereforematerial properties.

DuPont and GE use proprietary manufacturing technologies to producesheet. There are five classes of glass fiber reinforced thermoplasticsheet available today--three from GE: "Azdel", "Azmet" and "Azloy" andtwo from DuPont: "LDF" and "XTC".

All classes of these sheet materials have three features in common.

1. All are based on thermoplastic resins.

2. All contain short fibers varying between 0.5" to 3.0" long which canbe carbon, Kevlar or glass.

3. All have their materials more or less uniformly distributedthroughout the sheet.

As stated above, thermoforming uses a vacuum to stretch heat softenedthermoplastic sheet into all mold contours. These materials do notstretch adequately. In particular, stretching tears holes in the sheetand allows air to go through. This aborts stretching before a part iscompletely filled out. For thermoforming, resistance to "holing" is abasic sheet material property.

Surface properties degrade during stretching: glossiness, flatness andlevel of porosity is not competitive to that achieved by Open Molding.An inspection of product literature from GE and DuPont shows that allwere designed to be molded, even though they are sold in sheet form andare described as thermoformable. Traditionally, thermoforming uses avacuum and a one surface mold to make a part. Furthermore, the materialis softened for stretching. It is not melted for molding. The availablesheets require heated, matched metal molds and pressures up to 6,000psi. to force materials to flow.

All of the available materials fail by the same mechanism. Inparticular, when pulled, the hot sheet begins to stretch. Furtherstretching causes some fibers to pull apart (delaminate) from theirneighbor fibers. Delamination allows air to enter the sheet through thevoids between neighboring fibers. Holes develop in the hot sheet whenvoids between surfaces connect. Once a sheet has a hole, vacuum is lostand stretching stops before a part is finished being formed.

It therefore appears that delamination is a natural step inthermoforming any reinforced thermoplastic material as the fibers moveinto new positions. For the available composite sheet hole formingduring stretching is also natural particularly when stretching increasessheet area greater than about of 5%. Based on the above, it does notappear that the commercial materials available today are satisfactoryfor thermoforming.

U.S. Pat. No. 4,778,717 issued to me on Oct. 18, 1988, for a"Thermoplastic Thermoformable Composite Material" discloses and claims acomposite thermoplastic material which can be easily formed, and evenreformed if necessary, at elevated temperatures to desired complexshapes. The composite material is light and strong and is able to bethermally deformed, and even reformed, to desired shapes with relativelyminimal buckling or rippling. Because of the structure of the compositematerial of the 4,778,717 patent, there are limitations in the formingof large complex shapes without buckling, rippling or tearing since thematerial has very limited stretching abilities. U.S. Pat. No. 4,778,717is made of reference to provide a background for this invention and alsoto complete any disclosure in this application of the construction andformation of the composite material. In addition, reference is made to anumber of improvement patents to U.S. Pat. No. 4,778,717, all listing meas co-inventor. These are U.S. Pat. No. 5,236,776 issued on Aug. 17,1993; U.S. Pat. No. 5,338,600 issued on Aug. 16, 1994; and U.S. Pat. No.5,354,604 issued on Oct. 11, 1994.

The composite material of U.S. Pat. No. 4,778,717 is formed from a corematerial of a thermoplastic resin material and a pair of layers offabric material disposed on the opposite sides of the core material.Layers of a thermoplastic material envelope and impregnate the layers ofthe fabric material and bonds the layers of the fabric material to thecore. The layers of the fabric material have a total thicknesssufficient to impart strength and rigidity to the composite material.The core is of a sufficient thickness to provide for a shaping of thecomposite thermoplastic material at an elevated temperature to desiredshapes or configuration with relatively little rippling or buckling ofthe fabric material. The composite material of U.S. Pat. No. 4,778,717has received widespread acceptance for orthotics and shoe components.

SUMMARY OF THE INVENTION

This invention provides a thermoplastic thermoformable compositematerial which constitutes an improvement for particular uses over thecomposite material of U.S. Pat. No 4,778,717. In one embodiment of theinvention, a layer of fabric material is impregnated with thermoplasticthermoformable resin material to define a core of a fabric materialcovered with layers of resin material. The structure is sufficientlythick to provide for a shaping of the composite material at an elevatedtemperature to any desired configuration. The fabric material may beformed of glass, carbon or aramid and may be formed from chopped fibersor random strand mats. The fabric layer has a total thickness sufficientto impart strength and rigidity to the composite material.

In one embodiment of the invention, thicker layers of a thermoplasticthermoformable resin material also may be disposed on the opposite sidesof the fabric material. These thicker thermoplastic layers alsoimpregnate the fabric layer and provide a smooth external surface to thecomposite material. The thermoplastic layers are of a sufficientthickness to supply excess resin during thermoforming to flow into andplug any voids that form during thermoforming.

Other embodiments of the invention include additional layers of fabricand resin to form composite material of 5 or more layers of alternatingfabric and resin layers.

The present invention provides a solution to "holing" by redesigning thesheet as compared to the prior art including my prior patents. One wayis to laminate a thick layer (0.030" or greater) of pure resin to eachside of the composite. This makes a three layer sandwich. Excess resinis now available during delamination to flow into voids and plug them asthey form. If the composite material is to be formed into a final partwith light to no thermoforming, then the outer layers of resin coveringand impregnating the fabric core may be thinner (less than 0.030") sincea lessor amount of resin will be required to flow into any voids.

A working analogy for the present invention is found in tires thatresist going flat. These tires use a coating of flowable plasticmaterial applied on their inside walls. Material fills holes aspunctures occur and prevents air from escaping and the tire from goingflat.

In the drawings:

FIG. 1 is a fragmentary schematic perspective view of a first embodimentof the thermoplastic composite material of this invention in sheet form;

FIG. 2 is a fragmentary schematic perspective view of a secondembodiment of the thermoplastic composite material of this invention insheet form;

FIG. 3 is a fragmentary schematic perspective view of a third embodimentof the thermoplastic composite material of this invention in sheet form;and

FIG. 4 is a view schematically illustrating a method of forming thethermoplastic sheet material shown in FIG. 2.

Sandwich structures have been used previously by Medical MaterialsCorporation (MMC) the assignee of my previous patents to makethermoplastic composite sheet bend formable. Since 1987, MMC has beenselling carbon fiber containing sheet and parts to podiatry and footwearcustomers. In the present invention the use of a sandwich structure tomake a thermoplastic composite sheet thermoformable by stretching isunique when compared to the prior art including my previous patents.

Stretching uses the principal of conservation of volume. For example,when one sq. ft. of a thermoplastic having a suitable thickness ispulled in both the length and width direction, the surface areaincreases and the thickness decreases. The thickness decreasesproportionally to the increase in both length and width. This means thatthe volume of material in one square foot is the same after stretchingas before stretching. The sandwich structures of my prior patents cannot stretch acceptability because they were not designed to stretchsince the fabric layers were designed to slide relative to a coremember.

In particular, the prior art sandwich technology can not stretch sincethe sandwich will not elongate when pulled in the length or widthdirection because of the use of long fibers. Even if the fibers areshort this deficiency exists because there is no viable healingmechanism for hole repair. Specifically, the prior art sandwich cannotheal voids since this sandwich has the reverse construction. The priorart sandwich consists of two fabric layers, one on either side of athermoplastic core.

Because of the above, the prior art sandwich can make parts by bendforming only since the prior art sandwich uses a different mechanism tomake parts. When warm, this resin core softens and allows the skins toslide smoothly by each other in response to shaping stresses frombending. Sheets make wrinkle free parts for relatively small shapedapplications such as podiatry or footwear. Larger parts areprogressively more difficult to make wrinkle free.

The present application can be realized using a number of embodiments ofthermoplastic composite sheet sandwich products all of which stretch.

FIG. 1 illustrates a sandwich made by laminating a preform constructedfrom a single layer of prepreg or fabric 10. The prepreg is randomlychopped glass fibers impregnated with resin. In the case of choppedoriented prepreg this will be increased to two layers, where the glassfibers are positioned 0 and 90 degrees to each other. Thin layers 12 and14 of resin are formed on either side of the prepreg. The fabric layermay be in the order of 40 to 80 mils and the resin layers 12 and 14 maybe in the order of 10 to 30 mils each. This sheet construction may beused when the final part is formed with light to no thermoforming.

FIG. 2 illustrates a sandwich made by laminating a preform constructedfrom a layer of prepreg 10 and two layers 16 and 18 of solid resin sheethaving equal thickness, one on either side of the prepreg layer. Theprepreg or fabric layer may be in the order of 40 to 80 mils and theresin layers 16 and 18 may be in the order of 30 to 50 mils each. Thissheet construction may be used when the final part is formed with heavythermoforming.

FIG. 3 illustrates a sandwich made by laminating a preform constructedfrom two layers of prepreg 10 of the same composition, one on each sideof a thick solid sheet of resin 20. Two more sheets of solid resin 22and 24 of equal thickness to each other and about half the thickness ofthe center resin sheet are added, one on each side of the prepreg. Theprepreg or fabric layers may be in the order of 30 to 80 mils, the resinlayer 20 in the order of 60 to 120 mils and the layers 22 and 24 in theorder of 30 to 60 mils. It will be appreciated that additional layers offabric and resin may be added if desired to increase the stiffness andstrength of the final sheet construction.

The manufacturing process for making the thermoplastic compositesandwich sheet of the present invention is unique since the moreconventional methods do not work. The sheet is produced by the followingsteps illustrated in FIG. 4.

Step 1. The glass fibers 100 are prepared by chopping dry glass rovingand then dropping the chopped fibers onto a resin coated moving belt ofpolyethylene film 104. The resin is initially contained in a storagechamber 106 and is deposited as a layer 108 on the film 104. Fibers canbe varied in length from 0.25" on up to 4.00" and by weight. Also thefibers can be placed in the prepreg in two patterns, random or oriented.The prepreg or fabric layer 110 is thereby formed.

Step 2. Prepreg 110 is stored in a freezer 112 either as cut and stackedsheets or on rolls 114. Storage temperatures are to be kept below 15degrees F.

Step 3. Prepreg 110 is converted into cured sandwiches by laminatinglayers of resin 116 and 118. The conditions presented are approximate.Temperature is 220 degrees F. Cure time is about 30 minutes. Pressuresare time sequenced: p1=0 psi t1=5 min, p2=35 psi t2=3 min, p3=110 psip3=25 min. This is for making a 4'×8' panels at two per press opening.

Lay up

Preform construction relates to the product being made and threespecific examples of sandwich products are described above in FIGS. 1, 2and 3.

Thermoplastic composite sheet of the present invention can replace theindustry standard "fiberglass" which is a thermosetting materialconsisting of glass fibers and polyester resins. Structural propertiesof "fiberglass" are routinely modified by changing glass content andglass construction. As a point of comparison between the twotechnologies, samples of sheet of the present invention containingrandom glass and oriented glass fibers are compared to 0.125""fiberglass". This thickness is used in 40 to 50% of all "fiberglass"parts. Performance comparisons are shown below in Table 1 for sheetformed of oriented glass fibers.

                  TABLE 1                                                         ______________________________________                                        PERFORMANCE COMPARISONS                                                       (Fiber glass vs "Thermoplastic Composite Sheet (TCS)"                                                   "TCS".sup.2                                                                             TCS                                                     "Fiberglass".sup.1                                                                        (Oriented (Random                                   Properties    (Random FG) FG)       FG)                                       ______________________________________                                        Thickness (0.000 ins.)                                                                      0.125       0.100     0.135                                     Flexural str. (10.sup.3 psi)                                                                20,000      34,900    27,000                                    Flexural Mod. (10.sup.6 psi)                                                                1.40        2.34      1.30                                      Rigidity (lbs. in..sup.2)                                                                   240.4       229.7     225.0                                     Tensile Str. (10.sup.3 psi)                                                                 10,000      10,018    8,900                                     Tensile Mod (10.sup.6 psi)                                                                  1.40        1.39      --                                        Areal Density (lbs./ft..sup.2)                                                              1.0         0.8       0.90                                      ______________________________________                                         Notes:                                                                        .sup.1 Ref. for glass/polyester: Modern Plastics '92, p 406 column 6          (premix chopped glass). Glass content assumed, 30-35% by weight.              .sup.2 "TCS" test and methods are ASTM.                                       Oriented glass fibers                                                         1. stronger with higher modulus in bending at 0.100" thick than               glass/polyester is at 0.125" thick.                                           2. slightly less rigid in bending at 0.100" than glass polyester is at        0.125'.                                                                       3. is 20% lighter at approximately the same performance.                      Random Glass Fibers                                                           1. Is 5% to 10% ligher at approximately the same performance.                 2. Stronger and slightly thicker.                                        

In addition to the structural advantages described above, the presentinvention realizes a number of process benefits as follows:

1. Rapid fiberglass part fabrication--3 to 5 min./part starting withcold sheets as compared to hrs. for open molding.

2. Low labor content parts (as low as 15% of COG) with finished "asmolded" surfaces comparable to open molding.

3. Environmentally friendly sheet materials. Health and safety are notan issued for workers in thermoforming companies nor is the environmentor solid waste disposal.

4. Inexpensive tooling--low pressure process requires only (1) one toolsurface/part, made from low cost material.

5. New Products to Market Rapidly--tooling can be made rapidly.

Although this invention has been described with reference to particularembodiments, it is to be appreciated that various adaptations andmodifications may be made and the invention is only to be limited by theappended claims.

I claim:
 1. A thermoplastic thermoformable composite material forshaping and stretching into a desired form without voids and holes,includinga core including at least one layer of chopped fibers envelopedand impregnated by thermoplastic material to form a fiber layer core,layers of thermoplastic material respectively positioned on oppositesides of the fiber layer core, and the layers of thermoplastic materialbeing provided with a sufficient thickness to flow into and fill anyvoids or holes formed in the core as the composite material is shapedand stretched into a desired form.
 2. The thermoplastic thermoformablecomposite material of claim 1 wherein each of the layers ofthermoplastic material may have a thickness less than 30 mils (0.030")when the composite material is to provide a desired shape with a shallowdraw thermoforming to no thermoforming.
 3. The thermoplasticthermoformable composite material of claim 2 wherein each of the layersof thermoplastic material may have a thickness in the range of 10 to 30mils and the fiber layer core has a thickness in the range of 40 to 80mils.
 4. The thermoplastic thermoformable composite material of claim 1wherein each of the layers of thermoplastic material may have athickness greater than 30 mils (0.030") when the composite material isto provide a desired shape with a deep draw thermoforming.
 5. Thethermoplastic thermoformable composite material of claim 4 wherein eachof the layers of thermoplastic material may have a thickness in therange of 30 to 50 mils and the fiber layer has a thickness in the rangeof 40 to
 80. 6. The thermoplastic thermoformable composite material ofclaim 1 wherein the chopped fibers are at least one quarter inch (0.25")long.
 7. The thermoplastic thermoformable composite material of claim 6wherein the chopped fibers have a length in the range of 0.25 to 4.00inches long.
 8. The thermoplastic thermoformable composite material ofclaim 1 wherein at least a substantial number of the chopped fibers arepositioned in the same oriented direction in the fiber layer core. 9.The thermoplastic thermoformable composite material of claim 1 whereinthe chopped fibers are positioned randomly in the fiber layer core. 10.The thermoplastic thermoformable composite material of claim 1 whereinthe chopped fibers in the fiber layer core are selected from a groupconsisting of glass, carbon and aramid.
 11. The thermoplasticthermoformable composite material of claim 1 wherein the layers ofthermoplastic material are made from a material selected from the groupconsisting of acrylic, polycarbonate and ABS.
 12. A thermoplasticthermoformable sandwich construction, includinga fiber impregnated coreformed by chopped fibers enveloped and impregnated by thermoplasticmaterial and having first and second opposite flat surfaces, a firstlayer of thermoplastic material disposed on the first flat surface ofthe fiber impregnated core, a second layer of thermoplastic materialdisposed on the second flat surface of the fiber impregnated core, andthe first and second layers of thermoplastic material provided withsufficient thickness to flow into and fill holes or voids formed in thefiber impregnated core during thermoforming.
 13. The thermoplasticthermoformable sandwich construction of claim 12 wherein each of thelayers of thermoplastic material may have a thickness less than 30 mils(0.030") when the composite material is to provide a desired shape witha shallow draw thermoforming to no thermoforming.
 14. The thermoplasticthermoformable sandwich construction of claim 13 wherein each of thelayers of thermoplastic material may have a thickness in the range of 10to 30 mils and the fiber impregnated core has a thickness in the rangeof 40 to
 80. 15. The thermoplastic thermoformable sandwich constructionof claim 12 wherein each of the layers of thermoplastic material mayhave a thickness greater than 30 mils (0.030") when the compositematerial is to provide a desired shape with a deep draw thermoforming.16. The thermoplastic thermoformable sandwich construction of claim 15wherein each of the layers of thermoplastic material may have athickness in the range of 30 to 50 mils and the fiber impregnated corehas a thickness in the range of 40 to
 80. 17. The thermoplasticthermoformable sandwich construction of claim 12 wherein at least asubstantially number of the chopped fibers are positioned in the sameoriented direction in the fiber impregnated core.
 18. The thermoplasticthermoformable sandwich construction of claim 12 wherein the choppedfibers in the fiber impregnated core are selected from a groupconsisting of glass, carbon and aramid.
 19. The thermoplasticthermoformable sandwich construction of claim 12 wherein the layers ofthermoplastic material are made from a material selected from the groupconsisting of acrylic, polycarbonate and ABS.